1. Field of the Invention
The present invention relates to an amplifier circuit, particularly to a D class power amplifier circuit.
2. Background Information
Conventionally, in the field of digital audio technology, a technique of converting digital audio data (hereinafter to be referred to as PCM data) encoded by a PCM system into an analog signal using a D/A converter circuit has been used most commonly. Moreover, in driving a speaker using such analog signal, it has been typical that the analog signal is power-amplified by a power amplifier (hereinafter to be referred to as a power amp).
In recent years, most commonly used D/A converter circuits for use in an audio frequency band are the ones adopting a delta sigma modulation system (hereinafter to be referred to as a ΔΣ modulation system). Moreover, as for power amps, although analog amps of B class output structure are commonly used, power amps of D class output structure (hereinafter to be referred to as D class power amps) have been used more often particularly for use in where power efficiency is required.
Such D class power amp usually adopts a system in which an analog input is to be converted into binary data which is being pulse-width-modulated (e.g., Japanese Laid-Open Patent Application No. 2004-312594 (hereinafter to be referred to as patent reference 1)). In this regard, however, in the D/A converter circuit adopting the ΔΣ modulation system, an output signal is mostly being pulse-width-modulated (i.e., a pulse width modulation (PWM) process is done on the output signal). In the following, such output signal being PWM processed will be referred to as a PWM signal. By using this PWM signal as a driving signal for driving an output stage of the D class power amp, it is possible to construct a kind of full digital system D class power amp (e.g., Japanese Laid-Open Patent Application No. 2005-86611 (hereinafter to be referred to as patent reference 2)). In such full digital system D class power amp, since it is not necessary to have any analog signal intervene during signal processing, it is possible to simplify the circuit structure.
However, with respect to the conventional D class power amp, the amplitude of an analog signal, which is the final output signal, in an audio frequency band is supposed to be obtained by a product of a ΔΣ modulated signal and a power supply voltage VDD supplied to a D class output stage, and therefore, there is a problem in that fluctuations in the power supply voltage VDD will have an influence on the analog signal.
In order to solve such problem, it is necessary to inhibit fluctuations in the power supply voltage VDD. For this purpose, normally, a series regulator or a switching regulator is used as a power supply stabilizer. However, with respect to the series regulator, a difference between the supply voltage and the output voltage will result in a power loss. Therefore, there is a problem in that power efficiency will deteriorate. In the meantime, with respect to the switching regulator, although power efficiency is high, it is necessary to use a coil. Therefore, there is a problem in that the packaging volume and weight of a mounting base will be increased. Because of such factors, neither the series regulator nor the switching regulator is appropriate for use in a device such as a portable device that requires high power efficiency and reduced packaging volume and weight.
Now, referring to the above-mentioned patent reference 1, it discloses a technique for preventing an output signal from being influenced by possible fluctuations in a power supply voltage VDD within an analog system D class power amp. However, since the analog system D class power amp and the digital system D class power amp are different in terms of circuit structure, it is not possible to apply the technique disclosed in patent reference 1 to the digital system D class power amp.
In the meantime, referring to the above-mentioned patent reference 2, it discloses a technique for preventing an output signal from being influenced by possible fluctuations in a power supply voltage VDD by comparing the output signal with a predetermined reference signal and feedback-correcting the PCM data based on the comparison result. However, in this technique, it is necessary to arrange a comparatively large feedback loop including a ΔΣ modulation circuit, and therefore, there will be a long delay period, and stable operation of the base apparatus becomes difficult.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved amplifier circuit. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.